The nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1) is a member of the PARP enzyme family. This growing family of enzymes consist of PARPs such as, for example: PARP-1, PARP-2, PARP-3 and Vault-PARP; and Tankyrases (TANKs), such as, for example: TANK-1 and TANK-2. PARP is also referred to as poly(adenosine 5′-diphospho-ribose) polymerase or PARS (poly(ADP-ribose) synthetase).
Tankyrases (TANKs) were identified as components of the human telomeric complex. They have also been proposed to have roles in regulation of the mitotic spindle and in vesicle trafficking and they may serve as scaffolds for proteins involved in various other cellular processes. Telomeres, which are essential for chromosome maintenance and stability, are maintained by telomerase, a specialized reverse transcriptase. TANKs are (ADP-ribose)transferases with some features of both signalling and cytoskeletal proteins. They contain the PARP domain, which catalyses poly-ADP-ribosylation of substrate proteins, the sterile alpha motif, which is shared with certain signalling molecules and the ANK domain, which contains 16 to 24 ankyrin repeats, also present in the cytoskeletal protein ankyrin. The ANK domain interacts with a variety of different proteins, including the telomeric protein, Telomere Repeat binding Factor-1 (TRF-1). These proteins were therefore named TRF1-interacting, ankyrin-related ADP-ribose polymerases (TANKs).
One function of TANKs is the ADP-ribosylation of TRF-1. Human telomere function is regulated by a complex of telomere associated proteins that includes the two telomere-specific DNA binding proteins, TRF-1 and TRF-2. TRF-2 protects chromosome ends, and TRF-1 regulates telomere length. ADP-ribosylation inhibits the ability of TRF-1 to bind to telomeric DNA. This poly-ADP-ribosylation of TRF-1 releases TRF-1 from the telomeres, thereby opening up the telomeric complex and allowing access to telomerase. Therefore, TANKs function as positive regulators of telomere length, allowing elongation of the telomeres by telomerase.
Other roles for TANKs are suggested by the identity of proteins with which they interact—the insulin-responsive aminopeptidase, the Mcl1 proteins (which are members of the Bcl-2 family), the Epstein-Barr nuclear antigen-1, the nuclear and mitotic apparatus protein and the cytoplasmic and heterochromatic factor TAB 182- and its various subcellular localizations (nuclear pores, Golgi apparatus and mitotic centrosomes).
Tankyrase-2 (TANK-2) differs from tankyrase-1 (TANK-1) in that it lacks an N-terminal HPS domain (comprised of homopolymeric repeats of His, Pro and Ser residues), found in TANK1. However, it probably has some overlapping functions with tankyrase-1, given that both proteins have similar sub-cellular localizations, associate with each other and bind many of the same proteins.
PARP-1 is a major nuclear protein of 116 kDa consisting of three domains: an N-terminal DNA binding domain containing two zinc fingers, an automodification domain and a C-terminal catalytic domain. The enzyme synthesizes poly(ADP-ribose), a branched polymer that can consist of over 200 ADP-ribose units. The protein acceptors of poly(ADP-ribose) are directly or indirectly involved in maintaining DNA integrity. They include histones, HMG proteins, topoisomerases, DNA and RNA polymerases, DNA ligases, Ca2+- and Mg2+-dependent endonucleases and single-strand break-repair and base-excision repair factors. PARP protein is expressed at a high level in many tissues, most notably in the immune system, heart, brain and germ-line cells. Under normal physiological conditions, there is minimal PARP activity. However, DNA damage causes an immediate activation of PARP by up to 500-fold. The resulting poly(ADP-ribose) production has three consequences: first, DNA-damage-induced poly(ADP-ribosyl)ation of the N- and C-terminal tails of histone H1 and H2B or the selective interaction of these proteins with free or PARP-1 bound poly(ADP-ribose) contributes to the relaxation of the 30-nm chromatin fibre and increases the access to breaks; second, it signals the occurrence and the extent of DNA damage so that the cell can establish an adaptive response according to the severity of the injury (DNA repair or cell suicide); third, it mediates the fast recruitment of single-strand break-repair and base-excision repair factors.
Single strand breaks (SSBs) occur spontaneously in all cells. In the absence of PARP-1 activity these SSBs may be converted to double strand breaks (DSBs) during replication that can lead to collapse of the replication forks. DSBs are identified by their epigenetic mark, the phosphorylation of the core histone variant H2AX (γH2AX). The very rapid local decondensation of chromatin, which occurs in a γH2AX-independent manner at DSB's can be attributed to poly(ADP-ribose) production that is mediated locally by PARP-1.
Also developmental or environmental cues, such as steroids or heat shock, induce PARP-1 activation and the poly(ADP-ribose)-dependent stripping of histones from chromatin, thereby favouring the opening of the chromatin structure, which may allow transcriptional activation in the absence of DNA breaks.
Extensive PARP activation in cells suffering from massive DNA damage leads to severe depletion of NAD+. The short half-life of poly(ADP-ribose) results in a rapid turnover rate. Once poly(ADP-ribose) is formed, it is quickly degraded by the constitutively active poly(ADP-ribose) glycohydrolase (PARG), together with phosphodiesterase and (ADP-ribose) protein lyase. PARP and PARG form a cycle that converts a large amount of NAD+ to ADP-ribose. In less than an hour, over-stimulation of PARP can cause a drop of NAD+ and ATP to less than 20% of the normal level. Such a scenario is especially detrimental during ischaemia when deprivation of oxygen has already drastically compromised cellular energy output. Subsequent free radical production during reperfusion is assumed to be a major cause of tissue damage. Part of the ATP drop, which is typical in many organs during ischaemia and reperfusion, could be linked to NAD+ depletion due to poly(ADP-ribose) turnover. Thus, PARP or PARG inhibition is expected to preserve the cellular energy level thereby potentiating the survival of ischaemic tissues after insult.
As indicated above, the subcellular localization of several PARPs hints also to a physiological role of poly(ADP-ribosyl)ation in the regulation of cell division.
TANK-1 seems to be required for the polymerization of mitotic spindle-associated poly(ADP-ribose). The poly(ADP-ribosyl)ation activity of TANK-1 might be crucial for the accurate formation and maintenance of spindle bipolarity. Furthermore, PARP activity of TANK-1 has been shown to be required for normal telomere separation before anaphase. Interference with tankyrase PARP activity results in aberrant mitosis, which engenders a transient cell cycle arrest, probably due to spindle checkpoint activation, followed by cell death. Inhibition of tankyrases is therefore expected to have a cytotoxic effect on proliferating tumor cells.
PARP-1 and PARP-2 localize to centrosomes where they interact with kinetochore proteins. Ablation of the Parp-2 gene in mice causes significant DNA-damage-induced chromosome mis-segregation that is associated with kinetochore defects, which indicates that PARP-2 has a crucial guardian function in pericentric heterochromatin integrity. Furthermore PARP-1 associate with centrosomes linking the DNA-damage-surveillance network with the mitotic fidelity checkpoint.
The pivotal role of PARP in the repair of DNA strand breaks is well established, especially when caused directly by ionizing radiation or, indirectly after enzymatic repair of DNA lesions induced by methylating agents, topoisomerases I inhibitors and other chemotherapeutic agents as cisplatin and bleomycin. A variety of studies using “knockout” mice, trans-dominant inhibition models (over-expression of the DNA-binding domain), antisense and small molecular weight inhibitors have demonstrated the role of PARP in repair and cell survival after induction of DNA damage. The inhibition of PARP enzymatic activity should lead to an enhanced sensitivity of the tumor cells towards DNA damaging treatments.
PARP inhibitors have been reported to be effective in radiosensitizing (hypoxic) tumor cells and effective in preventing tumor cells from recovering from potentially lethal and sublethal damage of DNA after radiation therapy, presumably by their ability to prevent DNA strand break rejoining and by affecting several DNA damage signaling pathways.
U.S. Pat. No. 5,177,075 discusses several isoquinolines used for enhancing the lethal effects of ionizing radiation or chemotherapeutic agents on tumor cells. Weltin et al., (“Effect of 6(5-Phenanthridinone), an Inhibitor of Poly(ADP-ribose) Polymerase, on Cultured Tumor Cells”, Oncol. Res., 6:9, 399-403 (1994)), discusses the inhibition of PARP activity, reduced proliferation of tumor cells, and a marked synergistic effect when tumor cells are co-treated with an alkylating drug.
Reviews of the state of the art has been published by Li and Zhang in IDrugs 2001, 4(7): 804-812, by Ame et al in Bioassays 2004, 26: 882-883 and by Nguewa et al., in Progress in Biophysic & Molecular Biology 2005, 88: 143-172.
Loss of PARP-1 increases the formation of DNA lesions that are repaired by homologous recombination without directly regulating the process of homologous recombination itself. Familial breast cancer is commonly associated with inherited defects in one of the BRCA1 or BRCA2 alleles. BRCA1 and BRCA2 are important for homologous recombination. The remaining functional BRCA1 or BRCA2 allele can be lost in some cells, thereby contributing to tumorigenisis. Thus, the tumors that arise are BRCA1 or BRCA2 deficient (e.g. BRCA2−/−) whereas the somatic cells retain functional BRCA proteins (BRCA2+/−). Inhibition of PARP activity in a BRCA1- or BRCA2-defective background might result in the generation of DNA lesions normally repaired by sister chromatid exchange, causing chromatid aberrations and loss of viability. Only relatively low levels of PARP-1 inhibitors may be required to produce a therapeutic effect given the acute sensitivity of the BRCA-defective cells. This is another example of a case where inhibitors of a normally non-essential DNA repair protein can be used as a single agent to treat tumors.
According to a review by Horvath and Szabo (Drug News Perspect 20(3), April 2007, 171-181) most recent studies demonstrated that PARP inhibitors enhance the cancer cell death primarily because they interfere with DNA repair on various levels. More recent studies have also demonstrated that PARP inhibitors inhibit angiogenesis, either by inhibiting growth factor expression, or by inhibiting growth factor-induced cellular proliferative responses. These findings might also have implications on the mode of PARP inhibitors' anticancer effects in vivo.
Also a study by Tentori et al. (Eur. J. Cancer, 2007, 43 (14) 2124-2133) shows that PARP inhibitors abrogate VEGF or placental growth factor-induced migration and prevent formation of tubule-like networks in cell-based systems, and impair angiogenesis in vivo. The study also demonstrates that growth factor-induced angiogenesis is deficient in PARP-1 knock-out mice. The results of the study provide evidence for targetting PARP for anti-angiogenesis, adding novel therapeutic implications to the use of PARP inhibitors in cancer treatment.
The PARP inhibitors of the present invention also demonstrate anticancer activity linked to disruption of tubulin polymerization.
Tubulin is composed of a heterodimer of two related proteins called α and β tubulin. Tubulin polymerizes to form structures called microtubules. Microtubules are highly dynamic cytoskeletal elements and play a critical role in many processes in eukaryotic cells, including mitosis, cell mobility, cell shape, intracellular organelle transport and cell-cell interactions.
For proper cell division to occur, it is essential that microtubules are able to polymerize and depolymerize. Microtubules in the mitotic spindle are more dynamic than those in non-dividing cells, and thus can be targeted by agents that affect microtubule dynamics. By altering microtubule polymerization/depolymerization these agents affect mitotic spindle formation, arrest dividing cells in the G2/M phase of the cell cycle, and ultimately lead to apoptotic cell death. As neoplastic cells have high proliferation rates, they can be targeted by these antimitotic agents.
Three main classes of tubulin-binding drugs, namely colchicine analogues, Vinca alkaloids and the taxanes have been identified, each of which possesses a specific binding site on the β-tubulin molecules. Paclitaxel and related taxanes represent a class of drugs that stabilizes microtubules, a process that ultimately leads to the freezing of the microtubule structures so that they can not be restructured. Subsequent arrest at mitosis induces the apoptotic mechanism to cause cell death. The second class of compounds, the colchicine analogues, as well as several other compounds, bind to the same site on β-tubulin as colchicine and disrupt polymerization and microtubular formation. The third class of compounds, vinblastine and several other vinca-related drugs, bind to the Vinca-site and prevent microtubule formation and destabilize microtubules.
Tubulin is also a target for treating disease states that are dependent or result from the abnormal formation of blood vessels (neovascularisation) such as cancerous tumours. In these cases the cytoskeleton of the vascular endothelial cells are disrupted through depolymerization of microtubules, which results from inhibiting the polymerization of tubulin to form microtubules. Microtubule length is dependent on the rate of depolymerization versus polymerization. Depolymerising microtubules through inhibition of polymerization leads to a change in endothelial cell morphology, which than causes a blockage or shutdown in blood flow. In the case of cancerous tumours, blood flow to the diseased tissue is stopped, depriving the tumour from oxygen and nutrients leading to necrotic cell death. Neovascular systems are more sensitive to these agents because they are more dependent on microtubule cytoskeleton than normal, healthy vascular endothelial cells which are also supported by actin based cytoskeleton structures. For a number of tubulin polymerization inhibitors that target the colchicine binding site of tubulin, the vascular targeting modality can be achieved at a lower in vivo concentration than the antiproliferative modality. Thus, agents that target the colchicine binding domain of tubulin can be potentially dual mode agents i.e. antimitotic and antivascular.
There continues to be a need for effective and potent anti-cancer therapy that include efficacy against tumors that are currently untreatable or poorly treatable, efficacy against multi-drug resistant tumors and minimal side effects. The present invention provides compounds, compositions for, and methods of, inhibiting PARP activity and binding tubulin for treating cancer. The compounds and compositions of the present invention differ from the prior art in that they have a dual mode of action (PARP inhibition and tubulin binding). Furthermore they have a high TANK inhibitory activity resulting in enhanced anti-cancer effects making them in particular useful for single agent treatment. They are also useful in enhancing the effectiveness of chemotherapy and radiotherapy where a primary effect of the treatment with the compound is that of triggering cell death under conditions of DNA damage.